Vishay: A Short History of Automotive Transmissive Sensors and Their Evolution

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By: Jim Toal, Director, Regional Marketing – Americas, Vishay Semicondutor, Optoelectronics Group

The Pre-Triassic Period
At the dawn of the present era, transmissive sensors in automotive systems took the form of an emitter-detector pair in through-hole packaging that was wave-soldered to a printed circuit board. The emitter and detector were facing each other so that if anything came between them, the output current of the photodiode or phototransistor would change. This change would be relayed to a controller and something would happen: a motor would start or stop, an indicator light would turn on or off, or a bag of chips would fall to the bottom of a vending machine.

The Triassic Period
The position of the discrete components could be difficult to precisely control. One might be higher than the other or at a slight angle; the leads might be bent, or during handling they would become disoriented. This posed a problem for the system because the output current from the detector would vary from board to board. The controller was looking for a certain signal level and, without an exact orientation, it wouldn’t get it. An evolutionary leap was ushered in by Vishay, which molded the discrete components in a common plastic housing to ensure exact orientation. This required several different package versions, each with a different gap between the emitter and detector, with a photodiode or phototransistor output, and with a different lead bend for horizontal or vertical gaps. These sensors were (and are) called transmissive sensors or slotted interrupters.
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The Jurassic Period
With more and more board assemblies going with pure surface mount components, the leads of the through-hole packages had to be bent so they could also be surface mounted. The plastic used in the housings had to change in order to withstand a +260°C reflow solder temperature. Horizontal slotted packaging did not evolve, and retained the form of a through-hole package. While many suppliers stopped evolving at this point, Vishay continued to push the envelope of device capabilities.

The TCPT- and TCUT1300X01 Period
Automotive customers needed a transmissive sensor that could operate at higher temperatures and was qualified to AEC-Q101 standards. The molding compound of the emitter and detector limited the operating temperature. Based on the techniques used in Origami, Vishay designed a lead-frame based, custom formed sensor that used emitter and detector chips without lenses. With a gap of 3.0mm and tightly controlled chip placement, the operating temperature increased from a maximum of +85°C to +105°C. Following the Orwellian theory of one detector good, two detectors better, Vishay created a transmissive sensor with two detector windows. With two detectors, steering angle sensors could not only detect a code wheel but could also determine direction and speed, which is critical input to electronic stability control units.

The advantage of this sensor’s construction over standard slotted interrupters includes:
- Tighter tolerances of package outline dimensions and contact pads
- Tighter tolerance of optical axis
- Better co-planarity of contact pads for mounting to PCB

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The TCPT- and TCUT1350X01 Period
Ever-demanding automotive customers needed the sensor to operate at still higher temperatures, to work in near-engine compartments and harsh environments up to 125°C. With some inspired design changes, Vishay was able to manufacture transmissive sensors that met this specification. In addition, the typical output current was increased from 0.6mA to 1.6mA.

The TCPT- and TCUT1600X01 Period
Imagine the lowly knob on your dashboard which controls the radio volume or the menu of your control display. It is an appendage that commands little thought when not being turned or pushed. Yet lately Vishay Opto has given that knob a great deal of thought, especially that push function. Back in the day, designers could use the TCPT1300X01 sensor to determine the position of the knob but would have to design completely separate circuitry for the push. Not anymore. The transmissive sensor has evolved further to the TCPT- and TCUT1600X01 which has a deeper channel that enables design engineers to redesign their code wheel to include a push function. The channel still has a gap width of 3mm and two detector windows, but the depth has increased from 2.8mm to 4.5mm.

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The TCUT1630X01 Period
While the increased dome height of 4.5mm will be sufficient for some applications, automotive customers need the added capability to be able to change the resulting action depending on the position of the knob when it is pushed. To fully meet this requirement, a third detector and third window has been added. The overall size of the sensor has increased to enable this additional feature. While it is a 3-channel transmissive sensor, it is still an incremental encoder.

The Fully Evolved TCUT1800X01
The TCUT1800X01 is a 4-channel transmissive sensor designed for incremental and absolute encoder applications. The sensor combines two infrared emitters with four detector channels in a small, 5.5 x 5.85 x 7mm surface mount package. In combination with an application specific code wheel or strip, the sensor is ideally suited for a wide range of applications such as rotary switches, incremental turn switches, and speed and motion control systems. The integration of four channels into the automotive-qualified package also makes this sensor an excellent choice for more complex applications such as automotive steering wheel encoding, where multiple channels or channel redundancy is required.

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Depending on the application, the sensor can work as an absolute encoder or incremental encoder. The difference between both operating modes is shown in Figure 1. Used as an absolute encoder, the TCUT1800X01 provides up to 16 different binary states. The application can decode this binary code and can directly translate this information to know at which of the 16 different positions the object is located.

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Figure 1

A typical example for this could be climate control knobs in a car. The 16 positions can be used to turn on the air conditioning or heater, and select for up 16 levels of airflow. The 16 positions could be used to not only select temperature but blower speed at different combinations of air vent locations.

For applications requiring more than 16 stages, incremental encoding could be the solution. Unlike absolute encoding, incremental encoding does not provide an exact position. It can provide the relative distance the code wheel or strip moved and in which direction.

This information can be processed by a microcontroller counting up or down to virtually generate an unlimited number of stages.

The incremental encoder example shows transitions that occur every 45° of rotation. This is twice the resolution of dual channel sensors which have only 90° phase shift information. The sensor can also be used as a fail-safe in safety related applications. For example, Ch1 and Ch2 have a phase shift of 90° while Ch3 and Ch4 are used to sense the same phase shift. The sensor could be used in applications where two channels are used for incremental encoding and two channels are used for absolute encoding.

Samples are available for all the sensors mentioned in this article. The TCUT1600X01 will be introduced in November while the TCUT1630X01 and TCUT1800X01 will be released in January.

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